1. Field of the Invention
The present invention relates to a sound-field correcting apparatus for correcting sound fields formed by sounds output from speakers to space by, for example, a multichannel audio system or the like, and to a method therefor.
2. Description of the Related Art
When a listener listens to sounds output from a plurality of speakers by playing back audio signals in a multichannel audio system, sound fields (acoustic effects) that the listener experiences as differing depending on a change in balance (such as the sound arrival time from each speaker) in accordance with a listening environment, such as the structure of a listening room and the listening position of the listener to the speakers. This results in the inability of the listener in the listening position to experience appropriate sound fields depending on the state of the listening environment.
Such a problem noticeablely occurs in an environment such as the inside of an automobile. In the inside of the automobile, because the position of the listener is almost limited to a seat position, a distance between the listener and each speaker is unequal, thus causing the sound arrival time from the speakers to differ. This produces an unbalanced sound field. In addition, the inside of the automobile is relatively small and is an almost completely sealed state. Accordingly, reflected sounds reach the listener while being complexly combined, causing an unbalanced sound field. Moreover, due to limitation of the positions at which the speakers can be installed, it is less common to install the speakers so that the sounds directly reach the listener's ears. Also a change in sound quality caused thereby affects the sound fields.
Accordingly, implementation of sound-field correction is known so that, in a listening environment where an audio system is actually used, the listener can listen to good sound fields as close to the original sound source as possible. In this sound-field correction, for example, for the audio signal to be output from each speaker, a delay time is adjusted so that a time difference of sound reaching the listener's ears can be corrected.
To efficiently perform sound-field correction, for example, instead of a simple implementation in which the user (listener) performs adjustment based on audibility only, it is preferable to automatically perform the correction with an apparatus.
Specifically, at first, by using a sound-field correcting apparatus, acoustic characteristics in the listening environment are measured. Based on the result of this measurement, a signal processing parameter for sound-field correction is set in an output system of the audio system. By using the speakers to output audio signals processed in accordance with the parameter set as described above, the listener can listen to the sound source in good sound fields corrected so as to adapt to the listening environment, without the user having to perform a particular sound field adjusting operation.
Regarding the measurement of the acoustic characteristics and sound-field correction based on the result of the measurement, for example, the following technique is known (see, for example, Japanese Unexamined Patent Application Publication No. 10-228286).
Firstly, in the listening environment, a microphone is disposed at a listening position (correcting position) corresponding to the position of the listener's ears. The sound-field correcting apparatus outputs measuring sounds from the speakers, uses the microphone to collect the output sounds, and converts audio signals based on the collected sounds from analog to digital form. The sound-field correcting apparatus obtains, for example, the distance information between each speaker and the listening position (the microphone-disposed position or a sound collecting position). Since the arrival time of sound in space from each speaker to the listening position is obtained based on the information of the distance, the sound-field correcting apparatus uses information of the arrival time from the speaker to set a delay time for an audio signal on a channel corresponding to the speaker so that sounds emitted from the speakers can reach the listening position with the same timing. In general, this correction is also called “time alignment”.